DNS Operations WG
Internet-Draft J. Jeong
R. Droms
R. Hinden
T. Lemon
M. Ohta
S. Park
S. Satapati
J. Wiljakka
Expires: November 2004 28 May 2004
IPv6 Host Configuration of DNS Server Information Approaches
draft-ietf-dnsop-ipv6-dns-configuration-00.txt
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Copyright Notice
Copyright (C) The Internet Society (2004). All Rights Reserved.
Abstract
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This document describes three approaches for IPv6 DNS server
address configuration. It details the operational attributes of
three solutions: RA option, DHCPv6 option, and Well-known anycast
addresses for Recursive DNS Servers. Additionally, it suggests
four deployment scenarios considering multi-solution resolution.
Therefore, this document will give the audience a guideline of IPv6
DNS configuration to select approaches suitable for their host DNS
configuration.
Table of Contents
1. Introduction..................................................3
2. Terminology...................................................3
3. IPv6 DNS Configuration Approaches.............................3
3.1 RA Option................................................3
3.1.1 Advantages.............................................4
3.1.2 Disadvantages..........................................5
3.1.3 Observations...........................................5
3.2 DHCPv6 Option............................................5
3.2.1 Advantages.............................................7
3.2.2 Disadvantages..........................................8
3.2.3 Observations...........................................8
3.3 Well-known Anycast Addresses.............................8
4. Interworking among IPv6 DNS Configuration Approaches..........9
5. Deployment Scenarios.........................................10
5.1 ISP Network.............................................10
5.1.1 RA Option Approach....................................10
5.1.2 DHCPv6 Option Approach................................11
5.1.3 Well-Known Addresses Approach.........................11
5.1.4 ISP Network for Home or SOHO Subscribers..............12
5.2 Enterprise Network......................................12
5.2.1 DNS Configuration in Multi-level Network Topology.....13
5.3 3GPP Network............................................13
5.3.1 Currently Available Mechanisms and Recommendations....14
5.3.2 RA Extension..........................................14
5.3.3 Stateless DHCPv6......................................15
5.3.4 Well-known Addresses..................................16
5.3.5 Recommendations.......................................16
5.4 Unmanaged Network.......................................16
5.4.1 Case A: Gateway does not provide IPv6 at all..........17
5.4.2 Case B: A dual-stack gateway connected to a dual-stack
ISP...................................................17
5.4.3 Case C: A dual-stack gateway connected to an IPv4-only
ISP...................................................17
5.4.4 Case D: A gateway connected to an IPv6-only ISP.......17
6. Security Considerations......................................17
7. Acknowledgements.............................................18
8. Normative References.........................................18
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9. Informative References.......................................19
10. Authors' Addresses..........................................20
Intellectual Property Statement.................................21
Full Copyright Statement........................................22
1. Introduction
IPv6 stateless address autoconfiguration provides a way to
autoconfigure either fixed or mobile nodes with one or more IPv6
addresses, default routes and some other parameters [3][4]. To
support access to additional services in the Internet that are
identified by a DNS name, such as a web server, the configuration
of at least one recursive DNS server for DNS name resolution is
also needed.
This document describes three approaches of DNS server address
configuration for IPv6 host: (a) RA Option [5], (b) DHCPv6 Option
[6]-[8], and (c) Well-Known Anycast Addresses for Recursive DNS
Servers [9]. Also, it suggests applicable scenarios for four kinds
of networks: (a) ISP network, (b) Enterprise network, (c) 3GPP
network, and (d) Unmanaged network.
Therefore, this document will help the audience select approaches
suitable for IPv6 host configuration of recursive DNS server.
2. Terminology
This memo uses the terminology described in [3]-[9]. In addition,
a new term is defined below:
Recursive DNS Server (RDNSS) A Recursive DNS Server is a name
server that offers the recursive
service of DNS name resolution.
3. IPv6 DNS Configuration Approaches
In this section, the operational attributes of three solutions are
described in detail.
3.1 RA Option
RA approach is to define a new Neighbor Discovery (ND) option
called RDNSS option that contains a recursive DNS server address.
Existing ND transport mechanisms (i.e., advertisements and
solicitations) mechanisms are used. This works in the same way
that nodes learn about routers and prefixes, etc. An IPv6 host can
configure the IPv6 addresses of one or more recursive DNS servers
via RA message sent periodically by router or solicited by a Router
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Solicitation (RS) [5]. This approach has an issue that the DNS
information needs to be configured in the routers doing the
advertisements. The configura-tion of DNS server address can be
performed manually by operator or automatically DHCPv6 client
running on the router. When advertising more than one RDNSS
options, an RA message includes as many RDNSS options as DNS
servers. Through ND protocol and RDNSS option along with prefix
information option, an IPv6 host can perform its network
configuration of its IPv6 address and recursive DNS server
simultaneously [3][4]. The RA option for recursive DNS server can
be used on any network that supports the use of ND. RA approach is
light-weight especially in wireless environment where RA message is
used for IPv6 address autoconfiguration, such as cellular networks.
The RA approach is useful in environments where the addresses of
the recursive DNS server(s) is changing because the RA option
includes a lifetime field. This can be configured to a value that
will require the client to time out the entry and switch over to
another recursive server address [5].
The preference value of DNS server, included in RDNSS option,
allows IPv6 hosts to select primary DNS server among several
servers; this can be used for load balancing of DNS servers [5].
3.1.1 Advantages
The RA Option for RDNSS has a number of advantages. These include:
1) The RA option is a simple extension of existing ND/Autoconfig
mechanisms [3][4]. No new protocol mechanisms are needed and
extending an ND implementation to support this option should be
very simple.
2) This approach, like ND, works well on a variety of link types
including point-to-point links, point-to-multipoint, and multi-
point (i.e., LANs), etc. RFC2461 [3] states that there may be some
link type on which ND is not possible; on such a link, some other
mechanism will be needed for DNS configuration.
3) All of the information a host needs to run basic internet
applications such as email, the web, ftp, etc., can be performed
with the addition of this option to ND and address auto-
configuration. The use of a single mechanism is more reliable and
easier to provide than when the recursive DNS server information is
learned via another protocol mechanism. Debugging problems when
multiple protocol mechanisms are being used is harder and much more
complex.
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4) This mechanism works over a broad range of scenarios and
leverages IPv6 ND. It works well on links that are high
performance (e.g., LANs) and low performance (e.g., wireless LANs
and cellular networks). In the latter case, combining the
recursive DNS server information with the other information in the
RA, the host can learn all of the information needed to use most
Internet applications such as the web in a single packet. This not
only saves bandwidth where this is an issue, but also minimizes the
delay to learn the recursive DNS server information.
5) The RA approach could be used as a model for other similar types
of configuration information. New RA options for other server
addresses that are common to all clients on a subnet would be easy
to define. This includes things like NTP servers, SIP servers, etc.
3.1.2 Disadvantages
ND is mostly implemented in kernel part of operating system.
Therefore, if ND supports the configuration of some additional
services, such as DNS, NTP and SIP servers, ND should be extended
in kernel part and need kernel compilation. DHCPv6, however, has
more flexibility for extension of service discovery because it is
an application layer protocol.
3.1.3 Observations
The proposed RDNSS RA option along with IPv6 ND and Auto-
Configuration allows a host to obtain all of the information it
needs to access basic internet services like the web, email, ftp,
etc. This is preferable in environments where hosts use RAs to
autoconfigure their addresses and all hosts on the subnet share the
same router and server addresses. It is preferable because the
configuration information can be obtained from a single mechanism,
it does not add additional delay, and it uses a minimum of
bandwidth. Environments like this include homes, public WLAN hot
spots, public cellular networks, and enterprise environments where
no per host configuration is needed.
DHCPv6 is preferable where it is being used for address
configuration and if there is a need for host specific
configuration. Environments like this are most likely enterprise
environments where the local administration chooses to have per
host configuration control.
3.2 DHCPv6 Option
DHCPv6 [6] includes the "DNS Recursive Name Server" option, through
which a host can obtain a list of IP addresses of recursive DNS
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servers [8]. The DNS Recursive Name Server option carries a list
of IPv6 address of RDNSSes to which the host may send DNS queries.
The DNS servers are listed in the order of preference for use by
the DNS resolver on the host.
The DNS Recursive Name Server option can be carried in any DHCPv6
Reply message, in response to either a Request or an Information-
request message. Thus, the DNS Recursive Name Server option can be
used either when DHCPv6 is used for address assignment, or when
DHCPv6 is used only for other configuration information as
stateless DHCPv6 [7].
Stateless DHCPv6 can be deployed either using DHCPv6 servers
running on general-purpose computers, or on router hardware.
Several router vendors currently implement stateless DHCPv6 servers.
Deploying stateless DHCPv6 in routers has the advantage that no
special hardware is required, and should work well for networks
where DHCPv6 is needed for very straightforward configuration of
network devices.
However, routers can also act as DHCPv6 relay agents. In this case,
the DHCPv6 server need not be on the router - it can be on a
general purpose computer. This has the potential to give the
operator of the DHCPv6 server more flexibility in how the DHCPv6
server responds to individual clients - clients can easily be given
different configuration information based on their identity, or for
any other reason. Nothing precludes adding this flexibility to a
router, but generally in current practice, DHCP servers running on
general-purpose hosts tend to have more configuration options than
those that are embedded in routers.
DHCPv6 currently provides a mechanism for reconfiguring DHCPv6
clients. To do this, the DHCPv6 server sends a Reconfigure message
to the client. The client validates the Reconfigure message, and
then contacts the DHCPv6 server to obtain updated configuration
information. Using this mechanism, it is currently possible to
propagate new configuration information to DHCPv6 clients as this
information changes.
The dhc WG is currently studying an additional mechanism through
which configuration information, including the list of RDNSSes, can
be updated. The Lifetime Option for DHCPv6 [10], assigns a
lifetime to configuration information obtained through DHCPv6. At
the expiration of the lifetime, the host contacts the DHCPv6 server
to obtain updated configuration information, including the list of
RDNSSes. This lifetime gives the network administrator another
mechanism to configure hosts with new RDNSSes by controlling the
time at which the host refreshes the list.
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The dhc WG has also discussed the possibility of defining an
extension to DHCPv6 that would allow the use of multicast to
provide configuration information to multiple hosts with a single
DHCPv6 message. Because of the lack of deployment experience, the
WG has deferred consideration of multicast DHCPv6 configuration at
this time. Experience with DHCPv4 has not identified a requirement
for multicast message delivery, even in large service provider
networks with tens of thousands of hosts that may initiate a DHCPv4
message exchange simultaneously.
3.2.1 Advantages
The DHCPv6 option for RDNSS has a number of advantages. These
include:
1) DHCPv6 currently provides a general mechanism for conveying
network configuration information to clients. So configuring
DHCPv6 servers allows the network administrator to configure
recursive DNS servers along with the addresses of other network
services, as well as location-specific information like time zones.
2) As a consequence, when the network administrator goes to
configure DHCPv6, all the configuration information can be managed
through a single service, typically with a single user interface
and a single configuration database.
3) DHCPv6 allows for the configuration of a host with information
specific to that host, so that hosts on the same link can be
configured with different DNS recursive name servers as well as
other configuration information. This capability is important in
some network deployments such as service provider networks or WiFi
hotspots.
4) A mechanism exists for extending DHCPv6 to support the
transmission of additional configuration that has not yet been
anticipated.
5) Hosts in some environments are likely to need DHCPv6 for other
configuration information.
6) The specification for configuration of DNS recursive name
servers through DHCPv6 is available as an RFC.
7) Interoperability among independent implementations demonstrated
at TAHI and Connectathon.
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3.2.2 Disadvantages
The DHCPv6 option for RDNSS has a few disadvantages. These
include:
1) Update currently requires message from server (however, see
[10]).
2) Because DNS information is not contained in RA message, the host
must receive two messages from the router, and must transmit at
least one message to the router. On networks where bandwidth is at
a premium, this is a disadvantage, although on most networks it is
not a practical concern.
3) Increased latency for initial configuration - in addition to
waiting for an RA message, the client must now exchange packets
with a DHCPv6 server; even if it is locally installed on a router,
this will slightly extend the time required to configure the client.
For clients that are moving rapidly from one network to another,
this will be a disadvantage.
3.2.3 Observations
We are aware of some applications where it would be preferable to
put the recursive DNS configuration information into an RA packet;
for example, on a cell phone network, where bandwidth is at a
premium and extremely low latency is desired. The final DNS
configuration draft should be written so as to allow these special
applications to be handled using DNS information in the RA packet.
However, in the general case, on general-purpose networks,
stateless DHCPv6 provides significant advantages and no significant
disadvantages. Even in the case where bandwidth is at a premium
and low latency is desired, if hosts require other configuration
information in addition to a list of DNS recursive name servers or
if hosts must be configured selectively, those hosts will use
DHCPv6 and the use of the DHCPv6 DNS recursive name server option
will be advantageous.
3.3 Well-known Anycast Addresses
The approach with well-known anycast addresses is to set well-known
anycast addresses in clients' resolver configuration files from the
beginning, say, as factory default. Thus, there is no transport
mechanism and no packet format. There is no delay to get response
and no further delay by packet losses [9].
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If other approaches are used in addition, the well-known anycast
addresses should also be set in RA or DHCP configuration files from
the beginning, say, as factory default to reduce configuration
effort of users.
An anycast address is an address shared by multiple servers (in
this case, servers are recursive resolvers). Request from a client
to the anycast address is routed to a server selected by the
routing system. The selection can be simply based on routing metric
or policy based one. However, it is a bad idea to mandate "site"
boundary on anycast addresses, because, most users just do not have
their own servers and want to access their ISPs' across their site
boundaries. Larger sites may also depend on their ISPs or may have
their own recursive resolvers within "site" boundaries.
DNS clients have redundancy by having multiple resolvers that there
should be multiple well-known anycast addresses configured on
clients. There is no point to have multiple servers sharing an
anycast address on a single link.
Small ISPs will operate one recursive resolver at each anycast
address which is shared by all the subscribers. Large ISPs may
operate multiple recursive resolvers at each anycast address to
distribute and reduce load, in which case, boundary between servers
may be fixed (redundancy is still provided by multiple addresses)
or change dynamically. DNS packets with the well-known anycast
addresses are not expected to cross ISP boundaries, as ISPs are
expected to be able to take care of themselves.
Well-known anycast addresses can be combined with cryptographic
security such as TSIG or DNSSEC. However, there is no point to
avoid manual configuration of DNS when secret information (such as
a shared secret key or a public key of root zone) for the
cryptographic security must manually be configured (and updated
periodically).
4. Interworking among IPv6 DNS Configuration Approaches
Three approaches can work together for IPv6 host configuration of
DNS server.
For ordering between RA and DHCP approaches, O (Other stateful
configuration) flag in RA message can be used [5]. If no RDNSS
option is included and O flag is set on in RA message, an IPv6 Host
may perform DNS configuration through DHCPv6 [6]-[8].
The well-known anycast addresses approach fully interworks with the
other approaches. That is, the other approaches can remove
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configuration effort on servers by using the well-known addresses
as the default configuration. Moreover, clients preconfigured with
well-known anycast addresses can be further configured to use other
approaches to override the well-known addresses, if configuration
information from other approaches are available. That is, all the
clients should have the well-known anycast addresses preconfigured,
in the case where there are no other mechanisms available. In
order to fly anycast approach with the other solutions, there are
three options.
The first option is that well-known addresses are used as last
resort, when an IPv6 host cannot get DNS server information through
RA and DHCP.
The second is that an IPv6 host can configure well-known addresses
as the most preferable in its configuration file.
The last is that the well-known anycast addresses can be set in RA
or DHCP configuration from the beginning, say, as factory default
to reduce configuration effort of users. According to either RA or
DHCP mechanism, the well-known addresses can be gotten by IPv6 host.
5. Deployment Scenarios
Regarding DNS configuration on the IPv6 host, several mechanisms
have being considered in the DNSOP Working Group such as Router
Advertisement extension, DHCPv6 and well-known preconfigured
anycast addresses as of today, and this document is a final result
from the long thread.
Note: in the applicable scenarios, authors do not implicitly push
any specific approaches into the restricted environments. No
enforcement is in this scenario and all mentioned scenarios are
probable. The main objective of this work is to provide a useful
guideline as Informational RFC.
5.1 ISP Network
From the ISP aspect, the IPv6 PE (Provider Edge equipment)
configuration is very difficult task because each host connects
multiple CPE (Customer Premises Equipment) components to the
backbone network infrastructure and even more difficult because
configuration must be done remotely [11]. Three approaches for DNS
configuration will benefit ISP network.
5.1.1 RA Option Approach
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RA extension for recursive DNS server can be used to allow a host
to get its recursive DNS server as well as IPv6 prefix at the same
time through a new DNS option [5] within RA message when the host
is attached to a new subnet. For easy configuration on the ISP,
DNS information, unsolicited RA message including a new DNS option
can be delegated to its subnet periodically. Because an IPv6 host
must receive at least one RA message for stateless address
autoconfiguration and router configuration, the host could receive
RDNSS configuration information in that RA without the overhead of
an additional message exchange.
This approach is so valuable in the mobile scenario which must
receive at least an RA message for detecting a new network than
others although administrator must configure DNS information on the
routers. Secure ND [12] can provide extended security when using
RA message.
5.1.2 DHCPv6 Option Approach
DHCPv6 can be used for RDNSS configuration through the use of the
Recursive DNS Server option, and can provide other configuration
information in the same message with RDNSS configuration [6]-[8].
Particularly, most ISPs are widely using DHCPv4 to allocate dynamic
IPv4 addresses to their customers in the current Internet, so that
DHCPv6 can be may applied for the current Internet in the same way.
DHCPv6 DNS option is already in place for DHCPv6 as RFC 3646 [8]
and moreover DHCPv6-lite or stateless DHCP [7] is nowhere as
complex as a full DHCPv6 implementation. DHCP is a client-server
model protocol, so ISP can handle user identification on its
network intentionally, and also authenticated DHCP [13] can be used
for secure message exchange.
Major applicable environment is probably home network because most
DSL gateways have a DHCP server now, and many of them have some
sort of DNS cache or relay. For this operation, all users and ISP
delegating equipments need to have DHCP function.
5.1.3 Well-Known Addresses Approach
Well-known anycast addresses approach is also a feasible and simple
mechanism for ISP [9]. The use of well-known anycast addresses
avoids some of the security risks in rogue messages sent through an
external protocol like RA or DHCPv6. The configuration of hosts
for the use of well-known anycast addresses requires no protocol or
manual configuration, but the configuration of routing for the
anycast addresses requires intervention on the part of the network
administrator. Also, the number of special addresses would be
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equal to the number of DNS servers that could be made available to
subscribers.
5.1.4 ISP Network for Home or SOHO Subscribers
One usual model for an ISP customer network is to have a Home or
SOHO gateway at the edge of the customer network, which is
connected to the ISP edge device. DHCPv6 prefix delegation can be
used to assign and communicate the customer prefixes from the ISP
device to the Home or SOHO gateway.
Because most home or SOHO subscribers will not bother to have their
own DNS servers and will not configure any information, not even
that for cryptographic security, the information about RDNSSes
provided by the ISP can be communicated to the Home or SOHO gateway
through the prefix delegation message exchange. The Home or SOHO
gateway can then pass that RDNSS configuration information to the
hosts in the customer network.
Home or SOHO subscribers with PPP connectivity will not configure
any information beyond that required for PPP. They just rely on
their ISPs and the connections to the ISPs are secure. Therefore,
such most subscribers can just rely on local DNS servers provided
by their ISPs without any cryptographic security. Subscribers are
still free to have their own mechanism for better security with its
own configuration information.
5.2 Enterprise Network
Enterprise network is defined as a network that has multiple
internal links, one or more router connections, to one or more
Providers and is actively managed by a network operations entity
[14]. An enterprise network can get network prefixes from ISP by
either manual configuration or prefix delegation [15]. In most
cases, because an enterprise network manages its own DNS domains,
it operates its own DNS servers for the domains. These DNS servers
within enterprise network process recursive DNS name resolution
requests of IPv6 hosts. DNS server configuration in enterprise
network can be performed like in Section 4, in which three
approaches can be used together.
IPv6 host can decide which approach is or may be used in its subnet
with O flag in RA message. For the first option in Section 4,
well-known anycast addresses are used as a last resort when O flag
in RA message is set off and RDNSS RA option is not included. The
option needs IPv6 hosts to preconfigure the well-known anycast
addresses in their DNS configuration storage, e.g.,
/etc/resolv.conf in UNIX.
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When the enterprise prefers well-known anycast approach to the
others, IPv6 hosts should preconfigure the well-known anycast
addresses like in the first option.
The last option, a more convenient and transparent way, does not
need IPv6 hosts to preconfigure the well-known anycast addresses
because the addresses are delivered to IPv6 hosts through either RA
option or DHCPv6 option as if they were unicast addresses. This
way is most recommended for the sake of user's convenience.
5.2.1 DNS Configuration in Multi-level Network Topology
The enterprise will have multi-level network topology. A network
administrator can easily configure DNS server in each router if
(s)he uses DHCPv6 Client/Server and DHCPv6 DNS Option [6]-[8].
(S)he manually needs to configure DNS information only in top-level
router(s). The rest of routers below can automatically configure
DNS information through DHCPv6. In the case where ND is used for
address autoconfiguration, the RA Option for recursive DNS server
can be used for IPv6 host configuration of DNS server in each
network level. For redundancy and load sharing, well-known anycast
addresses can be used by IPv6 hosts through RDNSS RA option.
Therefore, this model for DNS configuration is convenient and
efficient to both network administrator and users.
5.3 3GPP Network
IPv6 DNS configuration is a missing part of IPv6 autoconfiguration
and an important part of the basic IPv6 functionality in the 3GPP
User Equipment (UE). Higher level description of the 3GPP
architecture can be found in [16], and transition to IPv6 in 3GPP
networks is analyzed in [17] and [18].
In 3GPP architecture, there is a dedicated link between the UE and
the GGSN called the Packet Data Protocol (PDP) Context. This link
is created through the PDP Context activation procedure [19].
There is a separate PDP context type for IPv4 and IPv6 traffic. If
a 3GPP UE user is communicating using IPv6 (having an active IPv6
PDP context), it can not be assumed that (s)he has simultaneously
active IPv4 PDP context, and DNS queries could be done using IPv4.
A 3GPP UE can thus be an IPv6 node, and it needs to somehow
discover the address of the DNS server. Before IP-based services
(e.g., web browsing or e-mail) can be used, the IPv6 (and IPv4) DNS
server addresses need to be discovered in the 3GPP UE.
Section 5.3.1 briefly summarizes currently available mechanisms in
3GPP networks and recommendations. 5.3.2 analyzes the Router
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Advertisement based solution, 5.3.3 analyzes the Stateless DHCPv6
mechanism, and 5.3.4 analyzes the Well-known Addresses approach.
Section 5.3.5 finally summarizes the recommendations.
5.3.1 Currently Available Mechanisms and Recommendations
3GPP has defined a mechanism, in which DNS server addresses can be
received in the PDP context activation (a control plane mechanism).
That is called the Protocol Configuration Options Information
Element (PCO-IE) mechanism [20]. It is also possible to use
Stateless DHCPv6 for receiving DNS server addresses (described in
section 5.3.3) [7][8]. The DNS server addresses can also be
received over the air (using text messages), or typed in manually
in the UE. Note that the two last mechanisms are not very well
scalable. The UE user most probably does not want to type IPv6 DNS
server addresses manually in his/her UE. The use of well-known
addresses is briefly discussed in section 5.3.4.
It is seen that the mechanisms above most probably are not
sufficient for the 3GPP environment. IPv6 is intended to operate
in a zero-configuration manner, no matter what the underlying
network infrastructure is. Typically, the DNS server address is
needed to make an IPv6 node operational - and the DNS configuration
should be as simple as the address autoconfiguration mechanism. It
must also be noted that there will be additional IP interfaces in
some near future 3GPP UEs, e.g., Wireless LAN (WLAN), and 3GPP-
specific DNS configuration mechanisms (such as PCO-IE [20]) do not
work for those IP interfaces. In other words, a good IPv6 DNS
configuration mechanism should also work in a multi-access network
environment.
From 3GPP point of view, the best IPv6 DNS configuration solution
is feasible for a very large number of IPv6-capable UEs (can be
even hundreds of millions in one operator's network), is automatic
and thus requires no user action. It is suggested to standardize a
lightweight, stateless mechanism that works in all network
environments. The solution could then be used for 3GPP, 3GPP2,
WLAN and other access network technologies. A light, stateless
IPv6 DNS configuration mechanism is thus not needed in 3GPP
networks only, but also 3GPP networks and UEs would certainly
benefit from the new mechanism.
5.3.2 RA Extension
Router Advertisement extension [5] is a lightweight IPv6 DNS
configuration mechanism that requires minor changes in 3GPP UE IPv6
stack and Gateway GPRS Support Node (GGSN, the default router in
the 3GPP architecture) IPv6 stack. This solution can be specified
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in the IETF (no action needed in the 3GPP) and taken in use in 3GPP
UEs and GGSNs.
In this solution, an IPv6-capable UE configures DNS information
via RA message sent by its default router (GGSN), i.e., RDNSS
option for recursive DNS server is included in the RA message.
This solution is easily scalable for a very large number of UEs.
The operator can configure the DNS addresses in the GGSN as a part
of normal GGSN configuration. The IPv6 DNS server address is
received in the Router Advertisement, and an extra Round Trip Time
(RTT) for asking DNS server addresses can be avoided.
If thinking about cons, this mechanism still requires
standardization effort in the IETF, and the end nodes and routers
need to support this mechanism. The equipment software update
should, however, be pretty straightforward, and new IPv6 equipment
could support RA extension already from the beginning.
5.3.3 Stateless DHCPv6
DHCPv6-based solution needs the implementation of Stateless DHCP
[7] and DHCPv6 DNS options [8] in the UE, and a DHCPv6 server in
the operator's network. A possible configuration is such that the
GGSN works as a DHCP relay.
Pros for Stateless DHCPv6-based solution are
1) Stateless DHCPv6 is a standardized mechanism.
2) DHCPv6 can be used for receiving other configuration information
than DNS server addresses, e.g., SIP server addresses.
3) DHCPv6 works in different network environments.
Some issues with DHCPv6 in 3GPP networks are listed below:
1) DHCPv6 requires an additional server in the network unless the
(Stateless) DHCPv6 functionality is integrated into an existing
router already, and it is one box more to be maintained.
2) DHCPv6 is not necessarily needed for 3GPP UE IPv6 addressing
(3GPP Stateless Address Autoconfiguration is typically used), and
not automatically implemented in 3GPP IPv6 UEs.
3) Scalability and reliability of DHCPv6 in very large 3GPP
networks
(with tens or hundreds of millions of UEs) may be an issue, at
least the redundancy needs to be taken care of. However, if the
DHCPv6 service is integrated into the network elements, such as
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router operating system, scalability and reliability is comparable
with other DNS configuration approaches.
4) It is sub-optimal to utilize the radio resources in 3GPP
networks
for DHCPv6 messages if there is a simpler alternative available.
a) Use of Stateless DHCPv6 adds one round trip delay to the case
in which the UE can start transmitting data right after the
Router Advertisement.
5) If the DNS information (suddenly) changes, Stateless DHCPv6 can
not automatically update the UE, see [21].
5.3.4 Well-known Addresses
Using well-known addresses is also a feasible and a light mechanism
for 3GPP UEs. Those well-known addresses can be preconfigured in
the UE software and the operator makes the corresponding
configuration on the network side. So this is a very easy
mechanism for the UE, but requires some configuration work in the
network. When using well-known addresses, UE forwards queries to
any of the preconfigured addresses. In the current proposal [9],
IPv6 anycast addresses are suggested.
IPv6 DNS configuration proposal based on the use of well-known
site-local addresses developed in the IPv6 Working Group was seen
as a feasible mechanism for 3GPP UEs, but opposition by some people
in the IETF and finally deprecating IPv6 site-local addresses made
it impossible to standardize it. Note that this mechanism is
implemented in some existing operating systems today (also in some
3GPP UEs) as a last resort IPv6 DNS configuration mechanism.
5.3.5 Recommendations
It is suggested that a lightweight, stateless DNS configuration
mechanism is specified as soon as possible. From 3GPP UE's and
networks' point of view, Router Advertisement based mechanism looks
most promising. The sooner a light, stateless mechanism is
specified, the sooner we can get rid of using well-known site-local
addresses for IPv6 DNS configuration.
5.4 Unmanaged Network
There are 4 deployment scenarios of interest in unmanaged networks
[22]:
1) A gateway which does not provide IPv6 at all;
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2) A dual-stack gateway connected to a dual-stack ISP;
3) A dual-stack gateway connected to an IPv4-only ISP; and
4) A gateway connected to an IPv6-only ISP.
5.4.1 Case A: Gateway does not provide IPv6 at all
In this case, the gateway does not provide IPv6; the ISP may or may
not provide IPv6. Automatic or Configured tunnels are the
recommended transition mechanisms for this scenario.
The case where dual-stack hosts behind a NAT, that need access to
an IPv6 Recursive DNS Server, cannot be entirely ruled out. The
DNS configuration mechanism has to work over the tunnel, and the
underlying tunneling mechanism could be implementing NAT traversal.
The tunnel server assumes the role of a relay (both for DHCP and
Well-known addresses approaches). However, the deployment model of
Stateless DHCP Server in a tunneled environment is not well
understood or may not be justified.
RA-based mechanism is relatively straightforward in its operation,
assuming the tunnel server is also the IPv6 router emitting RAs.
Well-known address approach seems also simple in operation across
the tunnel, but the deployment model using Well-known addresses in
a tunneled environment is unclear or not well understood.
5.4.2 Case B: A dual-stack gateway connected to a dual-stack ISP
This is similar to a typical IPv4 home user scenario, where DNS
config parameters are obtained using DHCP. Except that Stateless
DHCPv6 is used, as opposed to the IPv4 scenario where the DHCP
server is stateful (maintains the state for clients).
5.4.3 Case C: A dual-stack gateway connected to an IPv4-only ISP
This is similar to case A, except that the tunnel originates from
the dual-stack gateway instead of the host.
5.4.4 Case D: A gateway connected to an IPv6-only ISP
This is similar to Case B.
6. Security Considerations
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As security requirements depend solely on applications and are
different application by application, there can be no generic
requirement defined at higher IP or lower application layer of DNS.
However, it should be noted that cryptographic security requires
configured secret information that full autoconfiguration and
cryptographic security are mutually exclusive. People insisting on
secure full autoconfiguration will get false security, false
autoconfiguration or both.
In some deployment scenario [17], where cryptographic security is
required for applications, secret information for the cryptographic
security is preconfigured through which application specific
configuration data, including those for DNS, can be securely
configured. It should be noted that if applications requiring
cryptographic security depends on DNS, the applications also
require cryptographic security to DNS. Therefore, the full
autoconfiguration of DNS is not acceptable.
However, with full autoconfiguration, weaker but still reasonable
security is being widely accepted and will continue to be
acceptable. That is, with full autoconfiguration, which means there
is no cryptographic security for the autoconfiguration, it is
already assumed that local environment is secure enough that
information from local autoconfiguration server has acceptable
security even without cryptographic security. Thus, communication
between a local DNS client and a local DNS server has the
acceptable security.
For security considerations of each approach, refer to the
corresponding drafts [5]-[9].
7. Acknowledgements
This draft has greatly benefited from inputs by David Meyer and Rob
Austein. The authors appreciate their contribution.
8. Normative References
[1] S. Bradner, "Intellectual Property Rights in IETF Technology",
RFC 3668, February 2004.
[2] S. Bradner, "IETF Rights in Contributions", RFC 3667, February
2004.
[3] T. Narten, E. Nordmark and W. Simpson, "Neighbor Discovery for
IP Version 6 (IPv6)", RFC 2461, December 1998.
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[4] S. Thomson and T. Narten, "IPv6 Stateless Address
Autoconfiguration", RFC 2462, December 1998.
[5] J. Jeong, S. Park, L. Beloeil and S. Madanapalli, "IPv6 DNS
Discovery based on Router Advertisement", draft-jeong-dnsop-
ipv6-dns-discovery-01.txt, February 2004.
[6] R. Droms et al., "Dynamic Host Configuration Protocol for IPv6
(DHCPv6)", RFC 3315, July 2003.
[7] R. Droms, "Stateless Dynamic Host Configuration Protocol (DHCP)
Service for IPv6", RFC 3736, April 2004.
[8] R. Droms et al., "DNS Configuration options for Dynamic Host
Configuration Protocol for IPv6 (DHCPv6)", RFC 3646, December
2003.
[9] M. Ohta, "Preconfigured DNS Server Addresses", draft-ohta-
preconfigured-dns-01.txt, February 2004.
9. Informative References
[10] S. Venaas and T. Chown, "Lifetime Option for DHCPv6", draft-
ietf-dhc-lifetime-00.txt, March 2004.
[11] M. Lind et al., "Scenarios and Analysis for Introduction IPv6
into ISP Networks", draft-ietf-v6ops-isp-scenarios-analysis-
02.txt, April 2004.
[12] J. Arkko et al., "SEcure Neighbor Discovery (SEND)", draft-
ietf-send-ndopt-05.txt, April 2004.
[13] R. Droms and W. Arbaugh, "Authentication for DHCP Messages",
RFC 3118, June 2001.
[14] J. Bound et al., "IPv6 Enterprise Network Scenarios", draft-
ietf-v6ops-ent-scenarios-01.txt, February 2004.
[15] O. Troan and R. Droms, "IPv6 Prefix Options for Dynamic Host
Configuration Protocol (DHCP) version 6", RFC 3633, December
2003.
[16] M. Wasserman, Ed., "Recommendations for IPv6 in 3GPP
Standards", RFC 3314, September 2002.
[17] J. Soininen, Ed., "Transition Scenarios for 3GPP Networks",
RFC 3574, August 2003.
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[18] J. Wiljakka, Ed., "Analysis on IPv6 Transition in 3GPP
Networks", draft-ietf- v6ops-3gpp-analysis-09.txt, March 2004.
[19] 3GPP TS 23.060 V5.4.0, "General Packet Radio Service (GPRS);
Service description; Stage 2 (Release 5)", December 2002.
[20] 3GPP TS 24.008 V5.8.0, "Mobile radio interface Layer 3
specification; Core network protocols; Stage 3 (Release 5)",
June 2003.
[21] T. Chown, S. Venaas and A. Vijayabhaskar, "Renumbering
Requirements for Stateless DHCPv6", draft-ietf-dhc-stateless-
dhcpv6-renumbering-00.txt, March 2004.
[22] C. Huitema et al., "Unmanaged Networks IPv6 Transition
Scenarios", RFC 3750, April 2004.
10. Authors' Addresses
Jaehoon Paul Jeong, Editor
ETRI / PEC
161 Gajeong-dong, Yuseong-gu
Daejon 305-350
Korea
Phone: +82 42 860 1664
Fax: +82 42 861 5404
EMail: paul@etri.re.kr
Ralph Droms
Cisco Systems
1414 Massachusetts Ave.
Boxboro, MA 01719
USA
Phone: +1 978 936 1674
EMail: rdroms@cisco.com
Robert M. Hinden
Nokia
313 Fairchild Drive
Mountain View, CA 94043
USA
Phone: +1 650 625 2004
EMail: bob.hinden@nokia.com
Ted Lemon
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Nominum, Inc.
950 Charter Street
Redwood City, CA 94043
USA
EMail: Ted.Lemon@nominum.com
Masataka Ohta
Graduate School of Information Science and Engineering
Tokyo Institute of Technology
2-12-1, O-okayama, Meguro-ku
Tokyo 152-8552
Japan
Phone: +81 3 5734 3299
Fax: +81 3 5734 3299
EMail: mohta@necom830.hpcl.titech.ac.jp
Soohong Daniel Park
Mobile Platform Laboratory, SAMSUNG Electronics
416, Maetan-3dong, Paldal-gu, Suwon
Gyeonggi-Do
Korea
Phone: +82 31 200 4508
EMail: soohong.park@samsung.com
Suresh Satapati
Cisco Systems, Inc.
San Jose, CA 95134
USA
EMail: satapati@cisco.com
Juha Wiljakka
Nokia
Visiokatu 3
FIN-33720 TAMPERE
Finland
Phone: +358 7180 48372
EMail: juha.wiljakka@nokia.com
Intellectual Property Statement
The following intellectual property notice is copied from RFC3668,
Section 5.
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The IETF takes no position regarding the validity or scope of any
Intellectual Property Rights or other rights that might be claimed
to pertain to the implementation or use of the technology described
in this document or the extent to which any license under such
rights might or might not be available; nor does it represent that
it has made any independent effort to identify any such rights.
Information on the procedures with respect to rights in RFC
documents can be found in BCP 78 and BCP 79.
Copies of IPR disclosures made to the IETF Secretariat and any
assurances of licenses to be made available, or the result of an
attempt made to obtain a general license or permission for the use
of such proprietary rights by implementers or users of this
specification can be obtained from the IETF on-line IPR repository
at http://www.ietf.org/ipr.
The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary
rights that may cover technology that may be required to implement
this standard. Please address the information to the IETF at ietf-
ipr@ietf.org.
Full Copyright Statement
The following copyright notice is copied from RFC3667, Section 5.4.
It describes the applicable copyright for this document.
Copyright (C) The Internet Society (2004). This document is
subject to the rights, licenses and restrictions contained in BCP
78, and except as set forth therein, the authors retain all their
rights.
This document and the information contained herein are provided on
an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE
REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND
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EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT
THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR
ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A
PARTICULAR PURPOSE.
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